The Tracking and Data Relay Satellite System (TDRSS) is a constellation of geosynchronous satellites which are the primary source of space-to-ground voice, data and telemetry for the Space Shuttle. The satellites also provide communications with the International Space Station and scientific spacecraft in low-Earth orbit such as the Hubble Space Telescope. Integral to the design of the TDRSS class of satellites is an architecture that includes a multiple access (MA), S-band, phased array antenna. Among its capabilities, the MA system receives and relays data simultaneously from multiple lower data-rate users and transmits commands to a single user.
An enhanced MA array antenna element was proposed which has simultaneous circular polarization capability and increased beamwidth. If developed, simultaneous circular polarization capability (left hand circular polarization (LHCP) and right hand circular polarization (RHCP)) will be required.
The proposed design specifications for the enhanced MA antenna elements are set forth below. Two bandwidth requirements, for example, narrowband and wideband are included in the specification. The wideband specification includes both the system transmit and receive bands.
TDRSS enhanced MA antenna element specifications.
Narrowband frequency (GHz)2.2-2.3Wideband frequency (GHz)2.03-2.3Peak directivity (dBi)15Directivity at 20 degree cone≧11(dBi)Axial ratio (dB)>−5 dBPolarizationSimultaneous LHCP and RHCPReturn loss (dB)≦−20Isolation (dB)≦−10
Short backfire antennas are widely used for mobile satellite communications, tracking, telemetry and wireless local network applications due to their compact structure and excellent radiation characteristics. Typically these antennas consist of half-wavelength dipole excitation elements for linear polarization or crossed half-wavelength dipole elements for circular polarization. To achieve simultaneous dual circular polarization using the related art would require integrating a network of hybrid switching components which introduces significant losses as well as disadvantages as to cost reliability, etc.).
Helix antennas naturally provide circular polarization. However, achieving dual circular polarization requires placing two helix antennas with opposite helical windings side by side, or a dual feeding arrangement. Placing helical antennas in proximity to each other can be problematic in the sense that coupling of the electromagnetic waves of one antenna to the other can occur absent a separation structure which would add weight to the assembly.
An article entitled “Compact Coaxial-Fed CP Polarizer,” by B. Subbarao and V. F. Fusco, IEEE Antennas and Wireless Propagation Letters, Vol. 3, 2004, states: “ . . . we use a circular waveguide with metal post inserts . . . to obtain a CP wave from an LP input, a 90° phase shift must be induced in one of the orthogonal components E∥ or E⊥, of the linearly polarized wave E which is applied at 45° to the post arrangement . . . . This phase shift is obtained by introducing slightly different phase constants for E∥ or E⊥. These are introduced by metal rods of equal size and spacing positioned diametrically across the aperture of the waveguide section. An equivalent circuit for a simplified version of this type of arrangement given in [ ] suggests that the inductance of these posts, together with their capacitive coupling, is providing the E∥ component with an impedance matched high-pass equivalent circuit thus advancing the phase of this component relative to its orthogonal component which propagates at normal waveguide phase velocity. By judicious design E∥, E⊥ components can be made to have equal amplitudes, hence if the length of the differential phase delay is made to be 90°, the exit signal will be a circularly polarized wave.”
An article entitled “Short Backfire Antenna With Conical Back Reflector And Double Small Front Reflectors by A. A. Ahmed, Journal of Islamic Studies, (9:2, 49-52, 1996 discloses “a conical back reflector and double plane small front reflectors fed through an open-ended circular waveguide excited with the dominant TE11 mode.” and which “shows a relatively high gain (17.2 dB).” Another article entitled “Experimental Measurements Of The Short Backfire Antenna” by L. R. Dod, October 1966, NASA Goddard Space Flight Center, Greenbelt Md., Technical Manual X-525-66-490 states on page 3 thereof that: “[t]he short backfire antenna is a medium gain antenna (10-15 dB.) with low side and back radiation. The antenna can be cross-polarized for orthogonal linear or circular polarization . . . . The addition of a λ/4 rim on the large reflector is necessary for low back radiation . . . . The short backfire may also serve advantageously as an array element.”
Polarization of an electromagnetic wave is defined as the orientation of the electric field vector. In a transverse electromagnetic (TEM) wave, the electric field vector is perpendicular to the direction of travel and it is also perpendicular to the magnetic field vector. Linear polarization is commonly referred to as vertical or horizontal polarization depending on the orientation of the emitter with respect to some local frame of reference. If there are two orthogonal emitters and if they are out of phase then an elliptical pattern is traced by the tip of the electric field vector as a function of time on a fixed plane through which the combined electromagnetic wave passes. A special case of the elliptical polarization is circular polarization where the orthogonal components are equal in magnitude and 90° out of phase.
The present invention discloses a short backfire antenna in combination with a cylindrical waveguide which includes an orthomode transducer (OMT), septum and adjustable impedance screws (polarizers) enabling simultaneous propagation and/or reception of two oppositely oriented circularly polarized electromagnetic waves. None of the foregoing references disclose this unique assembly of features and functions.